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Crew members on board when the leak was discovered on Aug. 30 quickly located and sealed a tiny hole in the Russian Soyuz spacecraft attached to the space station. The leak created a slight loss of pressure, and space officials said the station has remained safe to operate.

Russian cosmonauts Oleg Kononenko and Sergei Prokopyev embarked at 1559GMT (10:59 a.m. EST) for what it is expected to be a six-hour spacewalk to inspect the Soyuz's outer surface. They plan to uncover the thermal insulation covering the patched hole and take samples for experts to study.

Kononenko, who arrived at the station earlier this month with NASA astronaut Anne McClain and David Saint-Jacques of the Canadian Space Agency, said in a pre-flight interview that the spacewalk would be strenuous.

"It's going to be challenging both physically and technically," he said.

Roscosmos chief Dmitry Rogozin said in September that the hole could have been drilled during manufacturing or in space. He didn't say if he suspected any crew members of creating it on purpose, but his comment caused some bewilderment.